It is known to concurrently provide feedback air/fuel control and monitor of catalytic converter efficiency in response to exhaust gas oxygen sensors positioned upstream and downstream of the converter. In one known approach, converter degradation is indicated by comparing a ratio of downstream to upstream sensor switching frequency to a reference value while the vehicle is operating over a predetermined time period. In another approach, a ratio of downstream to upstream sensor amplitude is compared during the predetermined time period.
The inventors herein have recognized numerous problems with the above approaches. For example, variations in an operator's driving habits over the predetermined time period may result in variations of test results and erroneous failure indications.